Vehicles have been developed to perform an automatic idle-stop when idle-stop conditions are met and to automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like.
Following an engine idle-stop, pressure may be maintained in the hydraulic line to enable transmission and driveline functionality and to reduce engine restart times. Hydraulic line pressure may be maintained using, for example, auxiliary electric motor driven pumps, accumulators, etc., until the mechanical transmission pump reaches full flow capacity. However, the operation of an auxiliary electric pump during engine idle-stop may degrade fuel savings while increasing component costs. Similarly, the cost and time involved in recharging an accumulator following discharge at restart may limit the engine's ability to perform successive restarts.
Thus, in one example, some of the above issues may be addressed by a method of controlling a vehicle system including an engine and a transmission fluid pump driven by the engine. In one embodiment, the method comprises, during an engine start, adjusting a speed ratio between the engine and the transmission pump between a first speed ratio and a second speed ratio, the pump rotating faster relative to the engine at the first speed ratio as compared with the second speed ratio.
In one example, a transmission mechanical pump may be coupled to the engine through a gearbox, such as a variable gearbox including at least one gear set. The gearbox may further include a first electric clutch and a second one-way clutch. By adjusting the operation of the clutches, the mechanical pump may be coupled to the engine with a first gear set, such that the pump is driven at a first speed ratio, or with a second gear set, such that the pump is driven at a second speed ratio. The gear ratios of the gear sets may be such that the pump rotates faster relative to the engine at the first speed ratio than at the second speed ratio.
During engine start, when the engine speed is below a threshold (such as below 400 rpm), and transmission pump flow capacity may be limited, an engine controller may close the electric clutch so that the mechanical pump is coupled to the engine at the first gear ratio via the electric clutch and a first gear set, and the pump is temporarily operated at the first speed ratio. In one example, when operated at the first speed ratio, the transmission pump speed may be higher than the engine speed. For example, the gearbox may be a two-speed gearbox enabling a 1:1 and a 1:10 speed conversion. Herein, when the electric clutch is adjusted to temporarily operate the pump at the first speed ratio, the first gear ratio of the gearbox may enable the transmission pump to be operated at 400 RPM when the engine speed is 40 RPM, thereby improving the flow capacity of the transmission pump at the onset of the engine start operation. Then, once the engine has reached the threshold speed, the electric clutch may be opened so that the mechanical transmission pump is coupled to the engine at the second gear ratio via the one-way clutch and a second gear set, and the transmission pump is operated at the second speed ratio with full flow capacity. In one example, when the electric clutch is adjusted to temporarily operate the transmission pump at the second speed ratio, the second gear ratio of the gearbox may enable the pump to be operated at 400 RPM when the engine speed is also at 400 RPM.
By improving the flow capacity of the pump at the time of engine restart, pressurized transmission fluid may be delivered to the transmission, thereby enabling an early engagement of transmission clutches when a vehicle start and launch is requested. Furthermore, the need to maintain hydraulic line pressure with auxiliary transmission pumps, such as an electric pump, is substantially reduced. By reducing the need for auxiliary transmission pumps, substantial energy and fuel savings may be achieved without affecting restart times, and while also reducing component degradation due to prolonged auxiliary pump use.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.